The present invention pertains generally to laser devices, and more particularly, to a novel dual-mirror mirror mount for polarizing light emitted from a laser without the use of an intra-cavity Brewster window.
As increased applications of lasers are found due to the unique high-energy, high-precision properties of the output beam from such devices, the use of lasers throughout many areas of technology is becoming increasingly ubiquitous. As known by those skilled in the art, a laser is a very high frequency optical oscillator constructed from an amplifier and an appropriate amount of positive feedback. Lasers are used as critical components in a number of industries, including optical telecommunications, medical surgery, and manufacturing.
A typical gas laser comprises a plasma tubule discharge chamber enclosing a gaseous medium. An arc discharge is established through the gaseous medium, which serves to ionize the gas, thereby forming a plasma and elevating the electron energy states to the level required for lasing action. As the electrons recombine to lower energy states, light is emitted via spontaneous emission. Typically, a pair of optical resonator mirrors seal the two ends of the plasma tube so that light emitted by the plasma oscillates between the optical resonator mirrors and is amplified as it passes through the gaseous medium to achieve a lasing action in a manner known by those skilled in the art.
In a simple gaseous laser plasma discharge chamber with a cylindrical symmetry, the light output from the laser is randomly polarized. Each individual cavity mode has a linear polarization at any one time. However, the overall laser output is a time-varying mix of modes of different polarization. As a result, the output beam appears to be non-polarized when integrated over a fairly short period of time. Although the beam intensity is fairly constant, if the application involves polarization-dependent optics, then a polarizing intra-cavity Brewster window is employed which introduces sufficient loss in the plane of s-polarization (defined by the mode whose polarization vector for the electric field is perpendicular to the plane of incidence) so that only p-polarized output (defined by the mode whose polarization vector for the electric field is parallel to the plane of incidence) is produced. This occurs when the Brewster window is positioned at a Brewster""s angle defined as:
2(b)=arctan(n)
where n is the index of refraction of the window material and the index of refraction on either side of the window is assumed to be exactly 1. The Brewster window acts as a partial polarizer that ensures partial reflectivity for S-polarization and nominally zero reflectivity for p-polarization. Thus, the Brewster window provides maximum transmission efficiency at a preferred orientation for the polarization within the laser. The use of Brewster angle window assemblies is a standard technique that has been in use for many years, and, prior to the present invention, was the standard polarization method in commercial use for gas lasers. Polarization in gaseous lasers is described in greater detail in xe2x80x9cLasers and Electro-Optics: Fundamentals and Engineeringxe2x80x9d by Christopher C. Davis, Cambridge University Press, 1996 (ISBN 0-521-30831-3), which is incorporated herein by reference for all that it teaches.
To facilitate a better understanding of the advantages conferred by the present invention, a brief description of a conventional helium-neon laser 10 will be first described in conjunction with FIG. 1. As illustrated, laser 10 includes a coaxial gas discharge chamber 12 defining a first end 2 and a second end 4 at opposite ends of the coaxial axis. Discharge chamber 12 comprises a concentric capillary bore 18 located coaxially therein. Typically, a support web 20 provides support to ensure centralization and better rotational stability of the capillary bore 18. A cylindrical cathode 16 is positioned coaxially within the first end 2 of the discharge chamber 12.
A first mirror mount assembly 40 is hard sealed to the first end 2. First mirror mount assembly 40 includes a steel mirror mount 42 brazed to end plate 38. A mirror substrate 44 is coated with a mirror coating 46 and hard-sealed to a mirror cup formed in the mirror mount 42 using a pre-formed glass frit 48. End plate 38 is sealed to the first end 2 of discharge chamber 12 via a glass-to-metal seal 34.
A second mirror mount assembly 50 is hard sealed to the second end 4 of discharge chamber 12. Second mirror mount assembly 50 includes a steel mirror mount 52 brazed to end plate 68. A mirror substrate 54 is coated with a mirror coating 56 and hard-sealed to a mirror cup formed in the mirror mount 52 using a pre-formed glass frit 58. In the illustrative embodiment, second mirror mount assembly 50 includes an optional polarizing Brewster window 66. Brewster window is positioned within the internal chamber of the mirror mount 42 and arranged at a Brewster angle with respect to coaxial axis of the capillary bore 18. End plate 68 is sealed to the second end 4 of discharge chamber 12 formed by the glass capillary bore 18 via a glass-to-metal seal 64.
The electrical anode 14 of the laser in this embodiment is formed by the steel mirror mount 58. Electrical contacts to the cathode 16 are provided by support bonding straps 36 bonded to the cathode 16 and to the end plate 38. In an illustrative 2 mW design, the resonator defined by the two mirrors 46 and 56 and the capillary bore 18 is typically of a hemispherical design with the bore diameter being 1.5 mm, mirror 54 being a flat mirror, and mirror 44 being a 30 cm concave mirror. The 30 cm concave mirror 44 is the output coupler which has a convex output radius to collimate the exiting radiation. Typical reflectivity for the high reflector is 99.9+%, while the output coupler 44 has a nominal 1% transmission.
An arc discharge is established by applying a voltage from a power supply (not shown) across the anode 14 and cathode 16. The arc discharge causes the gasses within the discharge chamber 12 to be ionized, forming a plasma thereby. As the ions decay to lower energy states, light radiation is emitted in a manner well-known to those skilled in the art, and amplified by the optical resonator formed by mirrors 44, 54 and capillary bore 18 such that a lasing action occurs.
The current prior art configuration of a polarizing Brewster window mirror mount assembly as exemplified by mirror mount assembly 50 of the gas laser 10 shown in FIG. 1 is problematic. Because the Brewster window 66 is configured to reside within the mirror mount, manufacture of the mirror mount 50 is difficult because of the need to clean both sides of the window 66 during manufacture, the need to precisely position the window 66 at the Brewster""s angle in order to prevent loss in efficiency (i.e., reduced power output) of the laser from deviation from the Brewster""s angle, and the care required to mount the window in order to avoid stressing the window.
Accordingly, a need exists for a new and improved technique for polarizing a laser beam without the use of an internal Brewster angle window integrated into the mirror mount.
The present invention is a novel method and apparatus for polarizing a laser beam without the use of a mirror mount with an internal integral Brewster window. In accordance with the method and apparatus of the invention, the present invention eliminates the Brewster window altogether and integrates two mirrors, one preferably at approximately 45xc2x0 with respect to the other, along the exterior of the mirror mount structure. The mirror mount structure is open at one end and has a hollow cavity therein. A pair of mirrors are hard-sealed to the mirror mount structure. The first mirror is partially reflective and the second mirror is maximally reflective. The second mirror is arranged at a predetermined angle N with respect to the first mirror such that a light beam entering said mirror mount structure follows a beam path hitting the first mirror, reflecting off the first mirror and hitting the second mirror, and then retro-reflecting back on itself along the beam path of the entering light beam.
Because polarization is achieved using external mirrors rather than an integral internal mirror mounted within the mirror mount chamber, the mirror mount assembly of the invention is easier to manufacture, thereby resulting in higher manufacturing yields. Furthermore, since the polarization is achieved without employing an internal Brewster window, the cleaning issues associated with the internal window are eliminated. In addition, the angle of the mirrors is adjustable by bending the entire mirror mount as a unit. This simplifies angle adjustment and reduces the amount of accuracy required for setting the angle during manufacturer, thereby reducing complexity and cost of manufacture, and increasing the transmission efficiency due to the ability to achieve lower intracavity loss.